Conventional bi-metal, thermally activated switches usually contain: (1) a bi-metal leg having self resistance to generate heat; (2) a resistor wound on the bi-metal to generate heat; (3) an additional resistor to generate heat; (4) another thermal source; all to enable the bi-metal to bend due to hot expansion or cold contraction so as to drive the bi-metal to move a contact thereon to open or close a circuit. Such switches have the following defects:
(1) After actuation, the bi-metal returns to its normal state due to the gradual dissipation of heat. Except for a mechanical linkage system, an open circuit cannot be maintained, as in a circuit breaker. PA1 (2) With a given bi-metal material and size, there are no means for adjusting the actuating current except of adjustment of the static pressure of the bi-metal. PA1 (3) They do not provide a lamp for indicating directly the switch status. PA1 (4) In switches having a resistor type of thermal source, the positions and distance of the resistor and the bi-metal are unstable so that quality control is difficult. PA1 (5) The recovery from heat is too short in time. If the switch is continuously overloaded, the switch life will decrease. PA1 (1) The switch enclosure is made of insulating material. The switch contacts are fitted with conductor legs extending outside the switch enclosure to serve as an interface for connecting wires. PA1 (2) The interface permits the connection of series or parallel resistors whereby the switch actuating current can be adjusted. PA1 (3) A holding resistor is provided to maintain a switch open or closed state after thermal actuation. The holding resistor can be made of material emitting light and heat to provide a lighted indication of switch status. Such material may comprise a thermal generating gas such as neon or a lamp filament.